1. Field of the Invention
The present invention generally relates to nuclear medicine, and systems for obtaining nuclear medicine images of a patient's body organs of interest. In particular, the present invention relates to a novel method and system for more accurately detecting the occurrence of valid scintillation events.
2. Description of the Background Art
Nuclear medicine is a unique medical specialty wherein radiation is used to acquire images that show the function and anatomy of organs, bones or tissues of the body. Radiopharmaceuticals are introduced into the body, either by injection or ingestion, and are attracted to specific organs, bones or tissues of interest. Such radiopharmaceuticals produce gamma photon emissions that emanate from the body. One or more detectors are used to detect the emitted gamma photons, and the information collected from the detector(s) is processed to calculate the position of origin of the emitted photon from the source (i.e., the body organ or tissue under study). The accumulation of a large number of emitted gamma positions allows an image of the organ or tissue under study to be displayed.
Emitted gamma photons are typically detected by placing a scintillator over the region of interest. Such scintillators are conventionally made of crystalline material such as NaI (Tl), which interacts with absorbed gamma photons to produce flashes of visible light. The light photons emitted from the scintillator crystal are in turn detected by photosensor devices that are optically coupled to the scintillator crystal, such as photomultiplier tubes. The photosensor devices convert the received light photons into electrical pulses whose magnitude corresponds to the amount of light photons impinging on the photosensitive area of the photosensor device.
Not all gamma interactions in a scintillator crystal can be used to construct an image of the target object. Some of the interactions may be caused by gamma photons that were scattered or changed in direction of travel from their original trajectory. Thus, one conventional method that has been used to test the validity of a scintillation event is to compare the total energy of the scintillation event against an energy “window” or range of expected energies for valid (i.e., unscattered) events. In order to obtain the total energy of the event, light pulse detection voltage signals generated from each photosensor device as a result of a single gamma interaction must be accurately integrated from the start of each pulse, and then added together to form an energy signal associated with a particular event. Energy signals falling within the predetermined energy window are considered to correspond to valid events, while energy signals falling outside of the energy window are considered to correspond to scattered, or invalid, events, and the associated event is consequently not used in the construction of the radiation image, but is discarded.
Another instance of inaccurate information may arise when two gamma photons interact with the scintillation crystal within a time interval that is shorter than the time resolution of the system (in other words the amount of time required for a light event to decay sufficiently such that the system can process a subsequent light event as an independent event), such that light events from the two gamma interactions are said to “pile up,” or be superposed on each other. The signal resulting from a pulse pile-up would be meaningless, as it would not be possible to know whether the pulse resulted from two valid events, two invalid events, or one valid event and one invalid event.
More specifically, when a gamma photon interacts with the detector of a nuclear medical imaging system, it causes an electrical output pulse to be produced that rises relatively quickly and then decays from its peak value exponentially. The rise time of the pulse is constrained mainly by the design of the camera electronics, while the exponential decay time is dependent on the detector material. For a conventional NaI scintillation crystal detector material, the decay time constant is on the order of 0.25 microseconds. After the expiration of about 4 time constants (i.e. about 1 microsecond), the output pulse for detection and processing purposes is gone, such that the system is ready to detect a new gamma interaction event.
In actuality, however, the “tail” of the output pulse exists for a much longer time as defined by a second decay time constant, but with a peak value that is so small that it is not detectable. Where there are many gamma interactions in a period of time that is short as compared with the longer decay time constant, the values of the pulse tails of such interactions are added together in the detector, thereby producing a DC shift of the detector output signal.
The value of a gamma interaction event is determined by integrating the pulse signal produced by a single gamma photon for about 1 microsecond (i.e., four decay time constants), over all photomultiplier tubes coupled to the scintillation crystal. That is, the signals produced by all photomultipliers are summed together in order to obtain a measure of the total energy of the event. This summed signal can be called Esum, which is proportional to the energy of the incident gamma that interacted with the scintillation crystal. Where there are many gamma interactions in a period of time that is short as compared with the longer decay time constant, the accumulated value of the slowly decaying tails of many prior events gets added to the integrated value of the current event being processed, and therefore produces an error in the integrated value of the event. This is known as pre-pulse pile-up.
Different solutions to the pulse pile-up problem are known in the prior art. One such solution involves the use of pile-up rejection circuitry, which either precludes the detector from processing any new pulses before processing has been completed on a prior pulse, or stops all processing when a pile-up condition has been identified. This technique addresses the problem of post-pulse pile-up, wherein a subsequent pulse occurs before processing of a pulse of interest is completed. Such rejection circuitry, however, may undesirably increase the “deadtime” of the imaging system, during which valid gamma events are being received but are not able to be processed, thereby undesirably increasing the amount of time needed to complete an imaging procedure.
Another known technique addresses the problem of pre-pulse pile-up, wherein a pulse of interest is overlapped by the trailing edge or tail of a preceding pulse or pulses as described above. This technique uses an approximation of the preceding pulse tail to correct the subsequent pulse of interest. Such approximation is less than optimal because it is not accurate over the entire possible range of pile-up conditions. Further, it requires knowledge as to the precise time of occurrence of the preceding pulse, which is difficult to obtain using analog signals. Additionally, this technique consumes a large amount of computational capacity.
DC-offset correction is also known, for example, from U.S. Pat. No. 5,847,395, incorporated herein by reference in its entirety. The '395 patent discloses the use of a flash analog-to-digital converter (FADC) associated with each photosensor device (e.g., photomultiplier tube (PMT)), and a data processor that integrates the FADC output signals, generates a fraction of a running sum of output signals, and subtracts the fraction from the integrated output signals to generate an adjustment signal to correct the output signals for baseline drifts. However, this solution does not accurately compensate for baseline shifts caused by accumulation of afterglow signals or slow decay pulse tails.
The '395 patent discloses two methods of baseline correction: for relatively slow time scale drift errors caused by electronics, a digital-to-analog converter (DAC) is used to generate pseudo-event signals that are applied to the FADC; the resultant output of the FADC is applied to an integration correction processor to generate an integration adjustment signal that is subtracted from actual event signals from the FADC. For relatively fast time scale drift errors, when the detector is not viewing light generated by incident gammas or X-rays, the FADC is strobed during time intervals between real events, so as to sample the zero baseline signal. A running average of the sampled baseline is maintained over a predefined number of samples, and this average is used to generate a correction signal COR that is then subtracted from the actual event output signals produced by the FADC. Neither of these methods takes into account actual baseline drifts caused by accumulation of gamma event afterglow.
Therefore, there exists a need in the art for a solution that improves the elimination of inaccuracies in event pulse value caused by pulse pile-up.